ci: run tests in cibuildwheel

Previously we weren't running tests against the built wheel. The wheel
build could be different from the build used in the tests workflow.

Let's test our wheels to add extra confidence about their integrity.

Author: indygreg

docs/news.rst

@@ -21,6 +21,10 @@ Version History
   fragile and there may be unwanted changes. We're optimistic that standardizing
   on uv (except for musllinux ppc64le and s390x where uv isn't available)
   will lead to more stability over time.
+* CI now runs tests against the wheels we distribute. Previously, we ran
+  tests against a separate build that was theoretically identical. But the
+  builds may have been subtly different, leading to preventable bugs in our
+  wheels. (Enabling this test coverage did not uncover any failures.)
 
 0.24.0 (released 2025-08-17)
 ============================

pyproject.toml

@@ -65,9 +65,25 @@ build-frontend = "build[uv]"
 # Turn compiler warnings into errors.
 environment = "ZSTD_WARNINGS_AS_ERRORS=1"
 
+test-command = [
+    "pytest -v --numprocesses=auto",
+    "PYTHON_ZSTANDARD_IMPORT_POLICY=cffi pytest -v --numprocesses=auto",
+]
+test-groups = ["dev"]
+test-requires = ["cffi"]
+test-sources = ["tests"]
+
 [tool.cibuildwheel.linux]
 before-all = "if [ -x /usr/bin/yum ]; then yum install -y libffi-devel; fi"
 
+[tool.cibuildwheel.windows]
+# Environment variable syntax is different on Windows.
+test-command = [
+    "pytest -v --numprocesses=auto",
+    "set PYTHON_ZSTANDARD_IMPORT_POLICY=cffi",
+    "pytest -v --numprocesses=auto",
+]
+
 # This is needed to suppress generation of license-file, which isn't
 # compatible with metadata version 2.1, which our pinned setuptools
 # version currently uses.